1. Field of the Invention
The present invention relates to filter devices, and more particularly to a filter device in which interdigital transducers (IDTs) are arranged on a major surface of a piezoelectric substrate to define surface acoustic wave resonators.
2. Description of the Related Art
In the related art, surface acoustic wave devices, which are filter devices, have been used as a radio frequency (RF) filter, an intermediate frequency (IF) filter, and a resonator for a voltage controlled oscillator (VCO) for cellular phones, a video intermediate frequency (VIF) filter for a television, and the like. Japanese Unexamined Patent Application Publication No. 2005-117151 discloses a configuration of such a surface acoustic wave device.
Japanese Unexamined Patent Application Publication No. 2005-117151 discloses a surface acoustic wave device that includes interdigital transducers (IDTs) that are formed on a major surface of a piezoelectric substrate, reflectors that are disposed on the opposite sides of each of the IDTs in a propagation direction of surface acoustic waves that propagate from the IDTs, and wiring electrodes each of which is electrically connected to a corresponding one of the IDTs and each of which is disposed at least in the propagation direction of the surface acoustic waves.
However, in a surface acoustic wave device of the related art including the surface acoustic wave device, which is disclosed in Japanese Unexamined Patent Application Publication No. 2005-117151, a surface acoustic wave that propagates across a major surface of a piezoelectric substrate transmits vibrational energy to wiring electrodes depending on power applied to IDTs, the structure of the surface acoustic wave device, or the like, and as a result, breaking of the wiring electrodes and a short circuit between the wiring electrodes may occur. More specifically, in a surface acoustic wave device of the related art, there has been a problem in that when vibrational energy is transmitted to wiring electrodes, electrochemical migration occurs between the wiring electrodes or whiskers are generated between the wiring electrodes, and as a result, breaking of the wiring electrodes or a short circuit between a wiring electrode and another wiring electrode occurs.
For example, in a surface acoustic wave device in which an LT (LiTaO3) substrate is used as a piezoelectric substrate, a leaky wave is used as a surface acoustic wave that propagates across a major surface of the piezoelectric substrate. In the case where this leaky wave propagates to reflectors that are disposed on the opposite sides of IDTs, upon application of power, wiring electrodes that are positioned outside of the reflectors generate heat as a result of vibrational energy generated by the leaky wave being applied thereto, the leaky wave propagates by passing through the reflectors, and breaking of the wiring electrodes or a short circuit between the wiring electrodes may occur. In particular, in the case where the wiring electrodes are formed of a metal film of Au, Al, or the like, breaking of the wiring electrodes or a short circuit may occur due to the vibrational energy generated by the leaky wave, which propagates across the piezoelectric substrate.